Axis, by Robert Charles Wilson
I really loved Spin, to which this is a sequel; in fact, Spin was the first Hugo winner that I actually voted for. Axis is not as good a book, but it's still a good enough read; a complete change of central characters, pursuing a quest up the back country of an unknown parallel world, with a lot more emotional depth than you usually get in a two-fisted adventure tale. Took me ages to get around to reading it, but I am glad I finally did.